Laminating systems for applying two sheets of plastic film on opposite surfaces of an article have been known for a number of years. Examples of such systems are disclosed in U.S. Pat. Nos. 3,309,983; 4,268,345; and, 4,090,911. In all of these systems, a pair of supply rolls supply two continuous webs of film between and into the nip of a pair of pressure laminating rolls which also receive the article therebetween.
Respective webs of laminating film have a heat-activated coating on one surface thereof and the system includes shoe-type heating members that are partially wrapped around the laminating rollers to heat the rollers. The respective webs of film are entrained over the arcuate shoes to heat the film sufficiently to activate the adhesive so that it becomes slightly fluent and the pressure laminating rollers bond the heated webs to the article, which is sandwiched therebetween. In this type of system, pull rollers are located downstream of the pressure rollers and maintain tension on the laminated article as it is being moved between cooling fans, located above and/or below the laminated article.
The shoe-type heating members usually are heated through electrical resisting heating elements that are either mounted in the shoes or adjacent the rear surface thereof. The shoes define generally arcuate surfaces over which the laminating film passes to provide extended contact with the film immediately prior to entering the nip of the laminating rollers and thus activate the adhesive.
Certain problems have been encountered with laminating systems of this type and the primary shortcoming is the unevenness of the heat produced by the shoetype member which results in varying temperatures across the entire width of the web. In addition, the partially-heated film immediately enters the laminating area from the heated shoes and does not allow for adequate transfer of the heat applied to the sheets to the adhesive and also does not allow for distribution of the heat across the sheets. It will be appreciated that uneven heat applied across the web of the sheet can produce either overheating or underheating of the web, which will result in a defective product.
Another problem encountered with the prior art systems is the fact that the laminating film must be of a certain character and, thus, the number of different laminating films that can be used is limited. For example, in the systems disclosed in the prior art, all of the heating of the laminating film is done by the stationary heated shoes immediately before the laminating film enters the nip between the pressure rollers. This is necessary to prevent stretching or elongation of the film in the heated condition, which may cause the film to wrinkle. This is particularly true when using a film such as polypropylene and, thus, these types of machines are incapable of laminating articles utilizing films other than polyethylene.
Thus, prior art laminating is effectively limited to using polyethylene films which can be heated by the stationary heated shoes of the type disclosed in U.S. Pat. No. 4,268,345 without stretching or wrinkling.
A further problem with the prior art types of machines is the fact that the speed is extremely limited since the film must be in contact with the heating shoes a sufficient amount of time to elevate the temperature thereof sufficiently to activate the adhesive. Thus, it is necessary to limit the speed of operation to obtain the proper heating characteristics, which may vary depending upon the adhesive and the type of polyethylene film that are being utilized.